COEALLINE FOEMATIONS. 



31 



sequently interrupted by large fluvial estuaries. Nor can they secure a footing on 

 too rapidly shelving rocks. Hence certain coasts which we should expect to be formed 

 of " living " coral are found to consist only of " dead " matter. The work is also 

 hindered or arrested altogether in certain storm-tossed seas, where the deeper and 

 colder waters are churned up and driven landwards. Thus may perhaps be 

 explained the absence of corals along a great part of the arid and parched seaboard 

 of Somaliland. 



But apart from these few interruptions, the shores and islands of the equatorial 

 zone are everj^where fringed or encircled by coral reefs. Besides the polypi, or 

 true coral builders, of which there are numerous species, other organisms also 

 secrete calcareovis matter, and thus contribute towards the enlargement of the dry 

 land. Account must also be taken of the seaweeds, algœ, nuUipores, and the like. 



Fig. 11.— Zone of the Coralline Islands. 



Scale 1 : 120.000.ono. 



Atolls. 



Barrier Reefs. Upheaved Lands. 



^^^^^— _ 1,800 Miles. 



some of which develop a solid crust on the rocky surfaces, like the lichens in 

 northern latitudes, while others accumulate in thick deposits on the beach. Being 

 thus gradually raised by the petrification of successive generations, the reefs con- 

 tinue to grow with the new life destined to disappear and become fossilised in its 

 turn. This growth of the living rock proceeds as a rule at an extremely slow rate, 

 not more than 38 or 40 inches in two hundred or three hundred years ; but the 

 field of operation is limited only by the boundless extent of the marine waters, and 

 the yearly result consequently represents hundreds of millions of cubic yards added 

 by all these zoophytes to the solid crust of the globe. 



Even islands situated in an area of depression and slowly subsiding when com- 

 pared with the surrounding sea-level may be fringed by a band of reefs growing at 

 a more rapid rate, and thus gradually rising above the surface of the water. The 



